1. Field of the Invention
The present invention relates to a package for use in transportation of an electric circuit board and from which an electric circuit board can be taken out and, more particularly, to a package capable of keeping an electric circuit board to have mounted thereon various kinds of electronic components in good condition during a time period from the time when the circuit board is only a substrate to the time when the circuit board is completed with various electronic components mounted thereon and from which package the circuit board can be taken out.
2. Description of the Prior Art
An electric circuit board has an electric circuit of conductive metal such as copper formed on a substrate made of resin, glass or ceramics. The electric circuit board has various kinds of electronic parts including semiconductor elements and other electronic components such as resistors, capacitors or reactors mounted thereon to connect them to each other. Usually, processes for manufacturing an electric circuit board provided with an electric circuit on the substrate are performed in a factory or company other than that where processes for mounting various kinds of electronic components on the electric circuit board is performed. Therefore, an electric circuit board having no electronic parts mounted thereon is merchandized and sold and is required to be stored temporally and transported in the market.
A conductive metal such as copper which is forming an electric circuit of the electric circuit board is easily oxidized by air, causing difficulties of a soldering in a later process and difficulties in functioning to achieve its desired electrical characteristics. Furthermore, a recognition mark is usually formed in a pattern by the conductive metal on the substrate at the same time when the electric circuit is formed thereon, which recognition mark is utilized, in a later process, to detect the circuit position on the substrate. However, if the conductive material is oxidized, the recognition mark of the conductive metal deforms and makes it difficult for an image recognition machine to detect the recognition mark correctly. Therefore it is important to prevent the conductive metal from being oxidized during storage or transportation of the electric circuit board.
A solder plating or pre-flux is applied to the electric circuit to cause it not to react with air, which is conventionally employed to prevent the electric circuit from being oxidized. In addition to the above described effect to protect the conductive metal against the oxidation by air, a solder plating and pre-flux application have the following advantages. In the solder plating application, it is not necessary to remove the solder plating from the electric circuit board for a later process because various kinds of electronic components are fixed to the circuit board by a solder connection in the later process. In the pre-flux application, the pre-flux functions in two ways: one is to prevent the electric circuit from being oxidized until the electric circuit board reaches a process to mount electronic components thereon, and the other is to promote the soldering ability in the later process to solder the electronic components to the electric circuit board.
However, the previous two methods for preventing the conductive metal of the circuit board from being oxidized have a bad effect on the later process. The solder plating method is executed by applying the solder plating all over the surface of conductive metal including an electric circuit and a recognition mark and results in a very high cost. When the electronic components and other parts are mounted on the circuit board by soldering at a later process, there appears a disadvantage that a soldering position is not determined exactly. This results in a miss-mounting of electronic components and other parts or degradation in the accuracy of the soldering position. The method using the pre-flux is usually carried out in a way that a desired solder pattern is formed on the pre-flux layer by a screen printing method. The electronic components and other parts are put on the pre-flux layer and are connected to the circuit by a solder refloat process. During the solder refloat process, the pre-flux between the surface of the circuit boards and the solder is heated to a boil and then the electronic components and other parts are separated from the surface of the circuit board. This results in a problem that it is not possible to ensure the soldering between the electronic components and the circuit board. When the method using the pre-flux is used for a circuit board having an electric circuit on both sides, another problem appears. While one side of the circuit board is initially subjected to the solder refloat process, the whole circuit board is heated to a higher temperature and then the pre-flux on the other side loses its flux function. When the other side of the circuit board is next subjected to the solder refloat process, the pre-flux on the other side does not act as a useful flux and degrades the soldering connection.
On the other hand, one may consider another method in which a circuit board is not subjected to a treatment for preventing the oxidation of the conductive metal. The oxidized conductive metal is reduced to metal by a flux having a strong reduction ability before a process of soldering electronic components and other parts in a way described above. However, such a flux generates a halogen compound during the soldering process. The halogen compound must be finally removed by a cleaning process using, for example, flon. This is not desirable in view of the environmental safety.